Whenever a source of power, such as a radio transmitter, delivers power to a load, the power is delivered most efficiently when the impedance of the load is equal to the complex conjugate of the impedance of the source. For two impedances to be complex conjugates, their resistances are equal, and their reactances are equal in magnitude but of opposite signs. Adjusting the source impedance or the load impedance, in general, is called “impedance matching.”
Impedance matching is the practice of attempting to make the output impedance of a source equal to the input impedance of the load to which it is ultimately connected, usually in order to maximize the power transfer and minimize reflections from the load. Impedance matching is particularly important in RF communications devices.